1. Field of the Invention
The present invention relates to a switching power supply apparatus which can be incorporated in a personal computer, a facsimile machine or the like.
2. Description of the Related Art
FIG. 7 shows the configuration of essential circuit components of an example of a switching power supply apparatus. The switching power supply apparatus has a transformer 3 and other circuit elements connected as shown in FIG. 7. The positive terminal of a dc input power source 1 and one terminal of an input capacitor 2 are connected to one terminal of a primary coil N1 of the transformer 3. A drain terminal of a main switching element (metal oxide semiconductor field-effect transistor (MOSFET)) 4 is connected to the other terminal of the primary coil N1. A source terminal of the main switching element 4 is connected to the negative terminal of the dc input power source 1 and to the other terminal (grounded terminal) of the input capacitor 2. A control circuit 5 for controlling switching of the main switching element 4 is connected to a gate terminal of the main switching element 4.
The anode of a rectifier diode 6 is connected to an output terminal of a secondary coil N2 of the transformer 3. The cathode of a rectifier diode 7 and one terminal of a choke coil 8 are connected to the cathode of the rectifier diode 6. One terminal of an output capacitor 9 is connected to the other terminal of the choke coil 8. The other terminal of the secondary coil N2 and the anode of the rectifier diode 7 are connected to the other terminal of the output capacitor 9. A load 10 is connected in parallel with the output capacitor 9.
In the switching power supply apparatus arranged as described above, when the main switching element 4 is turned on by the control operation of the control circuit 5, the energy of electric charge in the input capacitor 2 supplied from the input power source 1 is transferred by conduction via the primary coil N1 and the main switching element 4 and is output from the secondary coil N2. The flow of energy output from the secondary coil N2 is rectified by the rectifier diodes 6 and 7, passes the choke coil 8, is smoothed by the choke coil 8 and is output to the load 10.
When the main switching element 4 is turned off, the energy stored in the choke coil 8 in the on state is output to the load 10 by the conduction via the load 10 and the rectifier diode 7.
As described above, the switching power supply apparatus shown in FIG. 7 is a feedforward converter type (feedforward DC-DC converter type) circuit such that the energy of the dc input power source 1 is received by the input capacitor 2, and the flow of the electric charge energy in the input capacitor 2, extracted through the secondary coil N2 of the transformer 3 when the main switching device 4 turns on, is rectified and smoothed by the output circuit 14 formed by the choke coil 8 and the output capacitor 9, thereby obtaining output energy.
The control circuit 5 turns on the main switching element 4 with a predetermined switching period T, detects an apparatus output voltage V.sub.OUT applied to the load 10, and controls the on-off operation of the main switching element 4 by changing and controlling the switch-on period t of the main switching element 4 (i.e., the portion of the switching period T during which the main switching element is "on") so that the apparatus output voltage V.sub.OUT is maintained at a predetermined level, as described below.
For example, if the number of turns of the primary coil N1 is N.sub.1 ; the number of turns of the secondary coil N2 is N.sub.2 ; the switching period of the main switching element 4 is T; the switch-on period of the main switching element 4 is t; and the voltage of the input power source 1 (the charging voltage across the input capacitor 2) is V.sub.IN, the apparatus output voltage V.sub.OUT is shown by the following equation (1): EQU V.sub.OUT =(N.sub.2 /N.sub.1).multidot.(t/T).multidot.V.sub.IN( 1)
Since the number N.sub.1 of turns of the primary coil N1, the number N.sub.2 of turns of the secondary coil N2, the switching period T of the main switching element 4, and the voltage V.sub.IN of the input power source 1, shown in equation (1), are predetermined, it is possible to control and stabilize the apparatus output voltage V.sub.OUT by changing and controlling the switch-on period t of the main switching element 4.
This control for stabilizing the apparatus output voltage V.sub.OUT is performed as described below. When the apparatus output voltage V.sub.OUT is lower than a predetermined level, the control circuit 5 compensates for the decrement of the apparatus output voltage V.sub.OUT from the predetermined level by increasing the switch-on period t of the main switching element 4 (on-duty (t/T)) by the corresponding amount. Conversely, when the apparatus output voltage V.sub.OUT is higher than the predetermined level, the control circuit 5 compensates for the increment from the predetermined level by reducing the switch-on period t of the main switching element 4 (on-duty (t/T)) by the corresponding amount.
The voltage V.sub.IN of the input power source 1 of the above-described switching power supply apparatus is changed, for example, according to specifications of the apparatus such as a facsimile machine in which the switching power supply apparatus is incorporated. The charging voltage V.sub.2 across the input capacitor 2 is also changed with the change in voltage V.sub.IN of the input power source 1. The input power source 1 has a predetermined variable voltage range (the range in which the charging voltage V.sub.2 across the input capacitor 2 can be changed). The switching power supply apparatus is designed by setting the numbers N1 and N2 of the turns of the primary and secondary coils and other circuit constants according to voltages in the variable voltage range of the input power source 1.
However, the variable voltage range of the input power source 1 (the range in which the charging voltage V.sub.2 across the input capacitor 2 can be changed) is considerably large. If the charging voltage V.sub.2 across the input capacitor 2 is very high, it is necessary for the control circuit 5 to reduce the on-duty of the main switching element 4 to a very small value. The on-duty, however, cannot be reduced below a predetermined minimum on-duty value according to the circuit arrangement. In such a case, it is extremely difficult for the control circuit 5 to suitably control and stabilize the apparatus output.
The circuit arrangement shown in FIG. 7 has problems related to variations in the input capacitor charging voltage V.sub.2. The input capacitor 2 receives the input power source voltage V.sub.IN. The input capacitor voltage V.sub.2 (i.e., the same voltage as the input power source voltage V.sub.IN) is converted according to the turns ratio of the transformer 3, i.e., the ratio of the number N.sub.2 of turns of the secondary coil N2 to the number N.sub.1 of turns of the primary coil N1 (N.sub.2 /N.sub.1). This converted voltage is induced across the secondary coil N2 and is applied to the rectifier diode 7 while the reverse voltage across the secondary coil N2 is applied to the rectifier diode 6. Therefore, the rectifier diodes 6 and 7 must have a high enough peak inverse voltage to be adapted to use at the predetermined maximum input power source voltage V.sub.IN (charging voltage V.sub.2 across the input capacitor 2). Diodes having such a high peak inverse voltage have a large forward voltage drop and, hence, a considerably large power loss at the time of conduction. If such diodes are used, the power loss in the switching power supply apparatus becomes disadvantageously large (the circuit efficiency is reduced).
Moreover, if the input power source voltage V.sub.IN (the charging voltage V.sub.2 across the input capacitor 2) is high, the on-duty of the main switching element 4 is considerably short, as mentioned above. If the on-duty of the main switching element 4 is shorter, the effective value of the drain current flowing between the drain and the source of the main switching element 4 when the main switching element 4 is on becomes greater, so that the power loss in the main switching element 4 at the predetermined maximum input power source voltage V.sub.IN becomes disadvantageously large (the circuit efficiency is reduced).
When the main switching element 4 is off, the input power source voltage V.sub.IN (the charging voltage across the input capacitor 2) is applied between the drain and the source of the main switching element 4. Therefore, it is necessary for the main switching element 4 to have a high enough withstand voltage as to be adapted to use at the predetermined maximum input power source voltage V.sub.IN.
That is, a high-withstand-voltage device is used as the main switching element 4. Such a device has such a large drain-source parasitic resistance that there is a large loss of power in the main switching element 4 when the main switching element 4 is conducting. This is one of the causes of deterioration in the circuit efficiency of the switching power supply apparatus.
Another example of a conventional switching power supply apparatus is shown in FIG. 8. This example will be discussed below in connection with alternate embodiments of the invention.